Many machines, such as internal combustion engines, generate unwanted noise and vibration. The vibration typically comprises a fundamental component whose frequency is, for example, the potation frequency of the machine (this is called the first harmonic), plus one or more additional harmonics at frequencies that are integer multiples of the first.
Occasionally, the speed of a machine is more-or-less constant and it is then possible to reduce vibration caused by the machine with a "tuned damper", or "resonator" (the two terms are used interchangeably here). A tuned damper is a resonant system that is attached to a point where vibration is to be reduced, and it is built to resonate at or close to the frequency of the vibration. Purely passive tuned dampers have fixed characteristics, and will work only close to the frequency for which they are designed. At the resonance frequency, the input impedance of the resonator (defined as the ratio of a generalized force applied by the resonator at the point of attachment to a generalized velocity at the same point) will be exceptionally high (or in some cases, such as a Helmholtz resonator in a duct, exceptionally low: see the description in L E Kinsler, A R Frey, A B Coppens, J V Sanders "Fundamentals of Acoustics" 3rd ed. Wiley and Sons 1982 pp 241-242).
As an example of a tuned damper, a mass suspended on a spring will resonate at a characteristic resonance frequency. If the spring is attached to a structure vibrating at this resonance frequency, the amplitude of vibration of the structure at that point will be reduced (and as a consequence, the suspended mass will vibrate strongly). Similar effects can be described for acoustic resonators (e.g. Helmholtz resonators; L E Kinsler, A R Frey, A B Coppens, J V Sanders "Fundamentals of Acoustics" 3rd ed. Wiley and Sons 1982. pp 225-228) where sound pressure is the vibration to be reduced.
The disadvantage with passive tuned dampers is that they will work only close to one frequency, and if the frequency of the vibration drifts, the damper will cease to be effective. It is obvious that a damper whose characteristics can be adjusted with the frequency of vibration to be reduced would be an advantage. Implicit here is the concept that the period of the vibration varies only slowly, so that it takes many cycles of the vibration for the period of the vibration to change significantly (ie. a "quasi-periodic" system).
There have been many schemes for the adjustment of tuned dampers. Typically, the resonance frequency of the damper is changed by mechanically altering a stiffness or mass. The mechanism adjusting the damper senses the current frequency of the vibration and tries to adjust the damper resonance frequency to be the same. The main problem with such systems is that any error in the tuning of the damper can be extremely detrimental to performance, and there is no way of detecting this in a simple "open-loop" control system.
In more complicated systems, a "closed-loop" feedback control system is used to ensure that the damper is always kept close to resonance at the frequency of the unwanted vibration. These systems require additional sensors to make them work, but the performance is improved.
There is an important distinction between the systems described here, with resonators whose properties can be adjusted (called "adaptive-passive" systems) and so-called "active control systems". In an active control system, the outputs from the controller change on a timescale characteristic of the vibration itself, and these outputs usually drive actuators vibrating at the same frequency as the vibration (see, for example, P A Nelson and S J Elliott "Active Control of Sound" Academic Press 1992 and G B B Chaplin and R A Smith, U.S. Pat. No. 4,568,118, 1986). In the adaptive-passive system of the invention, the outputs of the controller vary on the much slower timescale characteristic of changes in the frequency of vibration. This reduces the computational requirements of the controller considerably. Furthermore, the power required to drive the actuators of an active control system is usually considerable, whereas in an adaptive-passive system, the power used to, adjust the resonator characteristics is usually negligible.
An ideal adaptive-passive system would be able to control components of vibration at several different frequencies (usually the first and subsequent harmonics of a quasi-periodic vibration), whilst maintaining the performance of the system as the frequencies change.